Make banlist collaboration easier by offering to preview effects before subscribing

[bkil]

• We already protect dozens of rooms based on an existing ban list.
• We would like to subscribe to a new ban list from another trustworthy operator.
• Before we subscribe, we would like to make sure that the new ban list will not have any adverse effect in our protected rooms.
• mjolnir should ask for a confirmation upon subscription if any existing users would be affected by new Server ACL or ban rules.

[bkil]

The same kind of confirmation or preview function should be provided in any other case when mjolnir starts moderating.

For example, when you protect a new room or after the permission level of the bot is promoted within a given room that allows it to complete a new type of action (i.e. banning or server ACL).

[bkil]

#240 is also related in that newly introduced ban rules could be tested against or grouped by the seniority of the affected user.

I.e., it might be less dangerous if a new ban rule only applied to users who joined recently or who have never spoken in the past, because those may very well be puppets themselves.